1. Field of the Invention
The present invention relates to an amplifier and more particularly to an amplifier suitable for driving a load which is operated by an AC signal, capable of being miniaturized and having improved characteristics.
2. Description of the Prior Art
FIG. 1 illustrates an IC (integrated circuit) referred to as an LA4520 which is described in "Semiconductor Handbook, Monolithic Bipolar Integrated Circuit Edited by Sanyo Electric Co., Ltd., '85" issued Mar. 20, 1985, p. 723.
The IC is used in a headphone cassette player only for reproduction, and comprises a pre-amplifier and a power amplifier in one package.
In FIG. 1, two sets of amplifiers each comprising an amplifier circuit 1 and feedback resistors 2 and 3 are provided in one package a. In addition, the package a is provided with input terminals 4 and 5 and an output terminal 6 corresponding to each of the amplifiers. Furthermore, a power supply input terminal 7 receiving a power supply voltage Vcc and a ground terminal 8 are provided therein. Each of the amplifier circuits 1 has a negative input, a positive input and an output, the feedback resistor 2 being connected between the negative input and the output. The feedback resistor 3 is connected between the negative input of each of the amplifier circuits 1 and the corresponding input terminal 4, and the positive input thereof is connected to the corresponding input terminal 5. The output of each of the amplifier circuits 1 is connected to the corresponding output terminal 6. A virtual grounding capacitor 9 is connected between each of the input terminals 4 and a grounding conductor. An input signal source 10 is connected between each of the input terminals 5 and the grounding conductor. A coupling capacitor 11 such as an electrolytic capacitor and a load 12 such as a headphone are connected in series between each of the output terminals 6 and the grounding conductor.
A small signal voltage applied from the input signal source 10 to the input terminal 5 is amplified by the amplifier circuit 1 and outputted from the output terminal 6. Closed loop gain of the amplifier circuit 1 is determined by the feedback resistors 2 and 3. In addition, the output bias voltage is generally set to 1/2 Vcc so that the amplitude of a signal component of the output voltage can be increased. The DC bias of the output voltage is removed by the coupling capacitor 11, so that only the amplified signal voltage is applied to the load 12. The coupling capacitor 11 stores charges during a positive half cycle of the amplified AC signal and discharges the charges during a negative half cycle thereof, so that electric power is stored in the form of voltage.
However, in the above described conventional amplifier, capacitance of the electrolytic capacitor externally provided, particularly, the outputting coupling capacitor 11, must be increased to improve the frequency characteristics. In order to supply power to a load such as a headphone even in the range of low frequency characteristics, capacitance of the coupling capacitor 11 must be approximately 220 .mu.F when the load resistance is 16.OMEGA. and the frequency is 45 Hz or the load resistance is 32.OMEGA. and the frequency is 22.5 Hz, for example. Therefore, the coupling capacitor 11 prevents manufacturing the miniaturized and light device. In addition, the frequency characteristics depend on the load. Furthermore, when the power supply is turned on, noise is produced by charging current for charging the coupling capacitor 11, which presents a problem when a load such as a headphone applied to ears or put in ears is driven.